Within the visual cortex of several mammalian species, more circuitry
is devoted to the representation of vertical and horizontal
orientations than oblique orientations. The sensitivity of this
representation of orientation preference to visual experience during
cortical maturation and the overabundance of cardinal contours in the
environment suggest that vision promotes the development of this
cortical anisotropy. We tested this idea by measuring the distribution
of cortical orientation preference and the degree of orientation
selectivity in developing normal and dark-reared ferrets using
intrinsic signal optical imaging. The area of the angle map of
orientation preference representing cardinal and oblique orientations
was determined; in addition, orientation selectivity indices were
computed separately for cardinal and oblique difference images. In
normal juvenile animals, we confirm a small, but statistically
significant overrepresentation of near horizontal orientations in the
cortical angle map. However, the degree of anisotropy did not increase
in the weeks that followed eye opening when orientation selectivity
matured; rather, it decreased. In dark-reared ferrets, an even greater
cortical anisotropy emerged, but angle maps in these animals developed
an apparently anomalous overrepresentation of near vertical
orientations. Thus, the overrepresentation of cardinal orientations in
the visual cortex does not require experience with an anisotropic
visual environment; indeed, cortical anisotropy can develop in the
complete absence of vision. These observations suggest that the role of
visual experience in cortical maturation is to promote the isotropic
representation of orientation preference.